Buffered photographic emulsion



. HUD- EFFECT OF ACID AND BASE ON W, 39- s. E. SHEPPARD 2,184,023

BUFFERED PHOTUGRAPHIC EMULSION Filed Jmw 9, 1956 m0 AMINO GROUPS BUFFER TYPICAL TITRATION CURVE FOR GELATIN 4 8 0 -(aMPARIs0II0r H/1N0 AMOUIVTOFAC'ID AND H BASEADDED ON'SPEED P 0F EMULSION I 6 0 400- CARBOXYL BUFFER X H CVURVE 4.0

-INC'REASE ACID 0 INCREASE BASE 40 FMS 6 0 840 I ca 0.1 NHCI PER @RAM cc 0/ N KOH SPE 2 SPEED AND PH or AN EMULSION GMT/N I000- -90 x WITHOUT BUFFER WITH BUFFER DIETHYI.

BARBITURIC ACID a0 P aoo Samuel E. Sheppard 6 Raymond H. Lambert A 20 I0 00 I0 20 E '1 I '1 I I W 500 cc. 0.INH2$04 cc. 0/ NNaOH BY 68 5 A/ F ATTORNEYS 1?T Dec. 19, 1939 BUFFERED PHOTOGRAPHIC EMULSION Samuel E. Sheppard and Raymond B. Lambert,

' Rochester, N. Y., asslg-nors, by mesne assign ments, to Eastman Kodak Company, Rochester, N. Y., a corporation of NewJersey Application June 9, 1936, Serial No. 84.317

5 Claims.

This invention relates to gelatins for photographic purposes and, particularly, to gelatinosilver halide emulsions containing buffering agents to maintain the hydrogen ion concentra- 5 tion constant.

It is known that gelatin has very poor buiiering action in the region most generally used and considered most desirable in photographic emulsion manufacture. This region extends between the limits of about pH 5 to about pH 10. Both above and below these limits the natural constituents of the gelatin afford a bufiering action. The photographic properties of an emulsion are in part dependent upon the hydrogen ion con centration or the emulsion. In the region desirable for photographic emulsion manufacture, a slight change in the hydrogen ion concentraiii , tion of the emulsion produces a great change.

in the photographic sensitivity or speed of the emulsion. A slight amount of a foreign acidic or basic substance, which may enter the emulsion or which may be formed in the emulsion during storage, may, therefore, result in a loss in sensitivity or a hypersensitizing which may be either general or local. Another factor which makes'the control of the hydrogen ion concentration desirable has resulted from the increased use of color sensitizing and hypersensitizing dyes which usiihlly involve the use of a high pH. In such emulsions the pH must be controlled with a great degree of accuracy. I It is, therefore, the principal object of the present invention to provide a means for obtaining a condition of constant hydrogen ion concentration in photographic emulsions.

We have found that this condition may be brought about by incorporating in the emulsion a emulsion with the addition of acid or base to unbuiiered emulsion and to an emulsion containingbuflering agents. p

- The hydrogen ion concentration of a solution is expressed by p which is the negative power of the'hydrogen ion concentration to the base 10, or the decadic logarithm of hydrogen ion concentration Addition of acid to the solution increases the hydrogen ion concentration, 1. e., reduces the value of pH. When acidis added too. partly neutralized solution of a weakly ionized acid, the pH change is slight. This resistance to change in the hydrogen .ion concentration is termed bufieringf It is exhibited by partly neutralized weak acids and bases and also by other compounds which dissociate hydrolytically to furnish acidic or basic ions. The pH at which neutralization of these weakly ionized compounds is half complete is termed the pK value of the compound. The subscripts a or b may be added to pK (as pm or pKa) indicating acidic or basic constants respectively. Since buders may be mono-,. di-'-, tri-, etcl, acidic or basic, the further subscripts 1, 2, 3, etc... may be added to indicate which acid or base gives the designated pK value. In this case pK is equal to the negative logarithm of the dissociation constant for acids (pH-=pK) and for bases PH==PKw-PKD, where 'pH is measured electro-' metrically and where Km=lih (dissociation constant of water), at about 25 0.- and Kw=3X 10- at about 40 C.

In the accompanying drawing wehave shown in Fig. i a typical titration curve for gelatin illustrating the change in pH with addition of acid or base to a solution of the gelatin. Gelatin is known to be made up of chains of condensed amino-acids in polypeptide c, with sidechain's of ionizable amino-acids. III the region below aboutpH 5 the gelatin exhibits a bufiering action, that is, a great increase in the amount of acid added toit results in only a slight change in the value of pH. This bufiering action has been attributed to the presence of the, carhoml groups in the gelatin. In the region above about H 8.5 a similar bufie action is noted on decrease 'of acid or increase of alkali in the gelatin. solution. The bufiering action at this point is thought to be due to the presence of aminogroups in the gelatin although only half as effective as in the acid region. In the region between pH and pH 8.5 the slope of the curve is great, indicating that a small change inthe amount of the acid or base added .to the gelatin solution results in a great change in the pH value of. the solution.

The significanceof this change in pH of gelatin, which is used to manufacture photographic emulsions, is illustrated in Fig. 2. This figure shows a curve for the change in sensitivity of a photographic emulsion, expressed as H. and D. speed, plotted against amount of acid or base added to the emulsion. It will be seen from this curve that the speed of a photographic emulsion in the region of pH 5 to pH is greatly afiected by the addition of a small amount of acid or base to the emulsion. Thm alteration of speed of an emulsion is, of course, highly objectionable.

Upon the addition of a small amount of a buffering agent to the photographic emulsion, the slope of the curve of Fig. 1 is materially decreased, thus showing that the addition-of acid or base to the emulsion results in less change in the pH of the emulsion. As indicated in Fig. 3, A is a curve for change in speed and pH of an emulsion, with addition of acid or base and will be seen tobe similar to the curve shown in Fig. l. The curve B of Fig. 3 illustrates the result of adding acid or base to a similar emulsion to which a buffering agent has been added. in'this case the buifering. agent consisting of diethylbarbituric acid (Veronal). It will be seen that in the case of the emulsion represented by the curve B. a comparatively large amount of acidic or basic material may be added to the emulsion without resulting in a great change in the pH of the emulsion. As shown in this figure, the change in pH of the emulsion with change in acid or base added to the emulsion is generally comparable to the change in speed. although the two are not straight line functions.

The bufl'ering agents wh ch may be used according to our invention must have certain deflnite characteristics. The bufl'ering action of chemical compounds of sufi'icie'nt solubility at a given pH depends upon their electrolytic and bydrolytic dissociation constants, on the first directly, as in the case of acids or bases, and on ,the second indirectly. in so far as saline compounds dissociating hydrolytically furnish basic or acidic materials or definite electrolytic d ssociation constants. The choice of suitable buffers in the required region of pH 5 to pH 10 is limiited to acids having pKa'values of about 5.5 to about 9.5 and to bases having pKb values of about 4.5 to about 8.5.

Further requirements oij-the builering agents used according to our invention are a sufliciently high water solubility so that they do not crystallize out in air-dry gelatin (water content 12-15%) and non-reactivity with silver halides, suclbas sulfiding or reducing tendencies. A further requirement is that the compounds should not be colored or. give colored products which would afiect the clarity of the emulsion.

Thefollowing bufiering agents have been found suitable for incorporation in photographic emulsicins. These are listed together with their PK va ues.

Iodoso benzoic acid A Trichloro- Diethyl-amino-cthyl alcohol I Bumnsm'e Aemn'rs.rK VALUEs PK P P P P P Disodium ph hate 7.3 Tricarballylic idnn 6. 1 Triethanolamine 113:. '5. 9 6. 52 Maleic acid 6. 2 Diethyl barbituric acid (Verona!) 7. 6 Dimethyl malonic acid 4.

Cyanuric acid. Pyromellitic acid d-Camphoric acid Parabanic acid Mucic acid--- Ethylene diamine 2:4 dichlorophenoia-Amino-n-butyric ac: Malonic acid Trichloro phenol Para-hydroxy benzaldchydn Accty acetone Acctonyl acetone Phloroglucinol- Resorcinol Guanidine- A ii an inin Dimethyl-bcnzylam1ne 'laurine Alpha-n'aphthol, Diethyl-amino-ethyl alcohol Assn-rs SUITABLE non "Burr'nnmo m FoLnowINo REGIONSPK VALUES a flBdiethyl-snccinic acid I t id- (gdoso J ac Taurinc Alpha-mph": nl

ss mwwm s s r sssae-ssa 3525883858888288$8 Although the compounds which may be used as buffering agents according to our invention are found to fall into a. number of well-defined chemical classes, such as acids, aldehydes, alcohols, etc., some of them may be grouped in more specificclasses. For example, among the polybasic organic acids are included malic acid, malonic acid, isopropyl malonic acid, diethyl malonicacid,

.dip'ropyl malonic acid, maleic acid, muclc acid,

tricarballylic acid, pyromellitic acid, d-camphoric acid and Bfl-diethylsuccinic acid. Among the heterocyclic' organic compounds there are included diethyl barbituric, acid, cyanuric acid,

parabanic acid and allantoin. Other compounds sseseseeeeeaeeasacs will listed above will also be found to'fall into well known groups.

As has been stated above, not all compounds which have the required pK value and function a as buffering agents are suitable for use in emulsions according to our invention Certain compounds which would be suitable for buffers for plain gelatin layers are undesirable for use in gelatino-silver halide emulsion layers by reason of their action on the sensitive silver salt. An example of a compound of this type is a hydroxylamine hydrochloride which has a suitable pK value but which has a reducing action and which tends to hyper-sensitize the emulsion. in Certain other compounds which are inert with respect to the silver salts are unsuitable due to the formation of colored substances. Such compounds may be used if this action is not objectionable but ordinarily these compounds are un-- suitable for use in emulsions. An'example of a compound of this type having a suitable pH value is para-nitrophenol.

The amount of buffering agent which may be used in the emulsion varies to some extent, particularly with respect to the compound used. We have found that a 0.01 molal solution in a gelatin or emulsion solution .is suitable. This amounts to 0.01 gram molecular weight per liter of a 5% solution of gelatin or emulsion, or about 1H5 0.15% of buffer based on the wet emulsion, that is, as emulsion containing90% of water. We are not limited to the use of this amount, however. The proportion may be as low as 0.05% or as great as 1.0% of buffer based on the wet emulsion, and depending, as stated above, on the solubility. The limits, as stated before, depend to some extent on the particular compound used. Certain of the compounds may not be used in excessive amounts or they will adversely affect the photographic properties of the emulsion.

We will now refer to a number of examples of emulsions bufiered according to our invention, it being understood that these examples are illustrative only.

Example I To 1000 grams of a gelatino-silver halide emulsion containing 80 to 85% of water there were added 2.5 grams of maleic acid and 2.5 grams of diethyl barbituric acid. This emulsion was.

0 then titrated with 0.1 normal potassium hydroxide and on titration it was found that the emulsion was buflered in the pH range of about 5 to 10..

Example If A buffer, diethyl barbituric acid, (0.4% by weight wet emulsion basis) .was added to an experimental emulsion while a check was made of emulsion from the same batch. Each was adjusted to the same pHQ An equal amount of acid was added to each and H. and D. speeds were obtained of the finished, coated and dried materials. The results are shown in the following table:

base were added to sets of each. H. and D. speed as well as other photographic characteristics were measured. It is quite apparent that speed is held quite constant for the bufiered emulsion but the unbuifered is remarkably affected. Fog was decreased in the presence of buffer.

- Example III As showing the bullet to have no harmful but a stabilizing effect on the coated emulsion, the following example is cited.

To an ordinary gelatino-silver halide negative emulsion there was added 1.2 grams of maleic acid per liter of 5% emulsion solution. The emulsion was then titrated with 0.1 normal po-. tassium hydroxide to a pH of 6.0 to 6.2 at which pH there was found to be little change in pH on further addition of alkali. The amount of alkali added will depend on the pH of the original emulsion mixture. A portion of this emulsion was coated on glass plates and was incubated for 72 hours at 120 F. in order to obtain an accelerated aging. The pH of the emulsion after coating was found to be about 6.24.

The incubated plates were then tested for photographic sensitivity or speed, for contrast or gamma, and for fog formation. Similar tests were made on buffered unincubated-emulsions and on unbuifered emulsions both incubated and unincubated. The unbufi'ered emulsions used for comparison had a pH of 5.72. The results obtained were as follows:

We have referred in the above examples to the total speed of the plates. This refers to speed or sensitivity of the plate when measured by the method of Hurter and Drifheld. Exposures were made in 9. Jones type 2-'B sensitometer, and the speeds determined as /2, where i is the inertia, that is, the point at which the straight line portion of the graph of D (density) against log E (logarithm of exposure in candlemeter-seconds) cuts the abscissa.-

In order to determine the gamma or contrast the plates, exposed in this sensitometer, were developed for a definite length of time in the following developer:

Water; liters" -,2.0 p-Methylaminophenol sulfate grams 8.8 Sodium 'sulphite, desiccated do 384.0 Hydroquinone do 35.2 Sodium carbonate, desiecated do 192.0 Potassium bromide do 20.0 Water to make liters 4.0

Gamma was determined in the usual way and was measured according to the H. and D. scale.

As is Well known, gamma according to this method is defined as the ratio of the density range of the negative to the range of the logarithm of the exposures producing it, that is,

where D1 and D2 arethe'minimum andmaximum The fog of the plates was measured by deter--,

mining the photographic density'of the unexposed plate and is expressed as such, i. e., as a density D= log T where T=percentage of light transmitted. i

The buffers used according to our invention may be incorporated in ordinary positive or negative gelatin emulsions or in emulsions designed for special uses, such as X-ray emulsions and printout emulsions. Various silver salts may be used as the sensitive agent, such as silver chloride, silver bromide or silver iodide.

The incorporation of buiiering agents in gelatin is not limited to gelatins used in the direct preparation of photographic emulsions, but to any gelatin used in contact with or in congunction with a photographic emulsion to form a photographic sensitive layer, as specifically described in the U. S. Patent No. 2,139,767, granted December 13, 1938, to Gale F. Nadeau.

For example, it is desirable that gelatin layers, in contact with the emulsion layer, viz., either above, below, or interstitial in multilayed coatings, should be at the same pH as the sensitive layer, for maximum stability thereof. Furthermore, gelatin layers applied at the back of a photographic film support, with the object of equaliz ing the moisture or water absorption of the emulsion layer, and thereby preventing curling of the film during or after processing, should be adjusted to much the same pH state as the emulsion. This will tend to equalize the rate of water absorption and change of dimensions of the emulsion layer and the backing layer, and therefore prevent distortion. Likewise, gelatin overcoating layers, such as gelatin layers coated over a sensitive emulsion for the purpose of protecting the sensitive emulsion, or preventing abrasion, may advantageously aieaoes be bufiered to prevent adverse action-on the sensitive layer. Y

What we claim is:

1. A gelatino-silver halide emulsion containing per liter approximately'0.02 gram-molecular weight of an organic compound selected -from the group consisting of maleic acid, malonic acid,

isopropyl malonic acid, diethyl malonic acid, di-

propyl malonic. acid, mucic acid, malic acid, tricarballylic acid, pyromellitic acid, d-camphoric acid, B S-diethyl succinic acid, diethyl barbituric acid, cyanuric acid, parabanic acid, and allan- 2. A gelatino-silver halide emulsion containing as a bufiering agent approximately 0.25% of maleic acid and approximately 0.25% of diethyl barbituric acid.

3. A gelatino-silver halide emulsion containing as a buffering agent approximately 0.02 grammolecular weight per liter of maleic acid. 7

45A gelatino-silver halide emulsion containing as a bufiering agent approximately 0.02 grammoiecular weight per liter of diethyl barbituric acid.

.5. The method of making a buflered gelatinosilver halide emulsion having a substantially constant pH within an appreciable portion of the region pH 5.5 to pH 9.5, which comprises adjusting the pH of the emulsion to the region pH 5.5 to pH 9.5, and adding to it an organic acid selected from the group consisting of polybasic and heterocyclic acids, inert with respect to silver halides, soluble in gelatin containing at least 12% of water, and having pK values afiording a buffering action in the region pH 5.5 to pH 9.5, V i said organic acid being present in suflicient amount to butter the emulsion in the region pH 5.5

to pH 9.5.

- SAMUEL E. SHEPPARD.

RAYMOND H. LAMBERT. 

